Um...for some reason, it would display when I followed the link, so I downloaded it and looked at it. The drawing is 40" x 20." Also keep in mind you will want sides and a bottom maybe. Is this for a stand alone CP or one you will mount on your cab?
Here is what I suggest. Print out this picture @ 100%. It will probably come out in at least 4 sheets of paper. Lay out all the sheets of paper, tape them together well, roll it up and scoot over to the lumber yard. Once your there, find out what they have in 3/4" pine. Is Plywood acceptable? My Home Depot has "half sheets" of Plywood and MDF for like $5 depending on what type of wood. Don't forget the bottom and sides though. You might get lucky, some lumber yards have knowledgable people there, and if you show them what you are doing, they might be able to help. Remember that treehouse commercial?
Normally, I would suggest contacting c0nsumer, but it seems he has only posted twice in the 1year plus since he registered. You could try to PM him anyways.
Sorry I couldn't answer your question directly.